Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the peanut butter, sugar, egg, vanilla extract, baking soda, and salt. Mix until smooth and well combined.
Gradually fold in the graham cracker crumbs and mini chocolate chips until evenly distributed.
Using a tablespoon, scoop out the dough and roll it into balls. Place them on the prepared baking sheet about 2 inches apart.
Flatten each ball slightly with the back of a fork.
Bake for 10-12 minutes or until the edges are lightly golden.
Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Once the cookies are cool, spread a generous layer of marshmallow fluff on the bottom of one cookie and top it with another cookie to create a sandwich. Repeat with the remaining cookies.